Your case comes with one 120mm mounted at the back
At the maximum, you can have 2 fans on the top of the case (these should be pushing air out, i.e upwards as hot air rises anyway), 2 on the main removable panel (left side panel if looking at the front of the case), one at the top in the back (already provided), and 1 on the bottom in the front, in front of the HDD bays.
At the minimum, I'd add the front fan. I'd say add the front fan (pushing air from outside to inside the case) and one top fan (pushing upwards).
The fan mounts look like this:
2 Top 120mm (or 140mm) fan mounts.
-2 Side 120mm (or 80mm [or 92mm]) fan mounts
-1 Rear 120mm (or 80mm [or 92mm]) fan mounts
-1 Front 120mm fan mount (With special screws specifically for that slot.)
Buy two 120 mm fans. Here are some quiet ones.
http://www.newegg.ca/Product/Product.aspx?Item=N82E16835226043
There are cheaper fans too, but make sure you check the noise ratings.